1. Field of the Invention
The present invention relates to a manufacturing method and a manufacturing apparatus in which two optical information boards are bonded together.
2.Description of the Related Art
In order to achieve high density of an optical disk, it is necessary to shorten the wavelength of a regenerative laser and to increase the numerical aperture (NA) of an objective lens. For the high-NA objective lens, however, the allowable disk tilt is very small. However, by decreasing the thickness of a board, the allowable range for the disk tilt is widened, so that the high density of a practical optical disk using a high-NA objective lens can be achieved.
For the optical disk using a thin board, two boards are bonded because one board hangs down by gravity. The bonding of two boards not only increases the mechanical strength but also doubles the capacity because both surfaces are used. A conventional method for manufacturing a disk will be explained with reference to FIG. 4 (Japanese Patent Application Laid-Open No. 8-161771). A board 6 provided with a first information signal surface on its one surface is manufactured by the injection molding method or the like method by using a transparent resin whose principal ingredient is polycarbonate. A reflective film is formed on the information signal surface by the sputtering method or the vacuum deposition method. Also, a board 9 provided with a second information signal surface on its one surface is manufactured by the injection molding method or the like method, and a reflective film is formed on the information signal surface by the sputtering method or the vacuum deposition method. For these reflective films, a metal whose principal ingredient is aluminum is used. While the board 1 is rotated at a low speed, a radiation-curing resin 1 is applied in a doughnut form (FIG. 4(A)). The second board 2 is lapped on the board 1 in such a manner that the reflective film on the information signal surface faces the radiation-curing resin 1 (FIG. 4(B)). By rotating the board 6 and the board 9 integrally at a high speed, the radiation-curing resin is spread uniformly between the board 6 and the board 9 (FIG. 4(C)). After the radiation-curing resin has been spread uniformly, radioacive rays (in FIG. 4, ultraviolet rays are used) are irradiated through the board 9 and the reflective film to cure the radiation-curing resin, whereby two boards are bonded together (FIG. 4(D)).
However, when the radiation-curing resin is applied in a doughnut form while the board is rotated, the other board to be bonded is lapped on the board, and the radiation-curing resin is spread between the two boards by rotating the boards integrally at a high speed, the radiation-curing resin at the inner peripheral portion is moved toward the outer periphery in large quantities by the centrifugal force, so that a variation in thickness of the radiation-curing resin occurs within the board.
This problem is generally solved by contriving the conditions such as the revolutions per minute, rotation time, and viscosity, application amount and application position of radiation-curing resin. Nevertheless, in the method using a rotating force, a disk is produced which has a tendency such that the radiation-curing resin is thinner at the inner peripheral portion than at the middle portion. Although this does not present a very serious problem when the radiation-curing resin has only a bonding function for bonding the two boards, for a disk requiring a function of an intermediate layer for regulating the distance between the two boards, it is difficult to stably form a highly accurate intermediate layer in the whole region of the disk because the accuracy required for the intermediate layer is on the order of micron.